feat: add display UI with splash screen, dashboard, and RSSI sparkline

Implement full SSD1306 OLED display support:
- Splash screen on boot (1.5s) with project name, version, board
- Dashboard showing radio state, config, counters, RSSI/SNR
- RSSI history sparkline (64-sample ring buffer, 2px-wide bars)
- DisplayOn/DisplayOff host commands routed via dedicated channel
- Pure render functions against generic DrawTarget (no hardware coupling)
- Display on/off control bypasses radio task entirely

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

vendor/heapless/tests/cpass.rs

@@ -0,0 +1,25 @@
+//! Collections of `Send`-able things are `Send`
+
+use heapless::{
+    spsc::{Consumer, Producer, Queue},
+    HistoryBuffer, Vec,
+};
+
+#[test]
+fn send() {
+    struct IsSend;
+
+    unsafe impl Send for IsSend {}
+
+    fn is_send<T>()
+    where
+        T: Send,
+    {
+    }
+
+    is_send::<Consumer<IsSend, 4>>();
+    is_send::<Producer<IsSend, 4>>();
+    is_send::<Queue<IsSend, 4>>();
+    is_send::<Vec<IsSend, 4>>();
+    is_send::<HistoryBuffer<IsSend, 4>>();
+}